This invention relates to laser arrays. A side-by-side array of n parallel identical injection single-mode waveguide laser elements with a close enough spacing to provide significant mutual evanescent coupling between adjacent elements, hereinafter referred to as an evanescent coupled array, has been found typically to have a higher lasing threshold for the zero order supermode in which the light is in phase in all the elements than for the (n-1).sup.th order supermode in which the light propagates down each element in antiphase with that propagating in its two immediately adjacent elements. The zero order supermode, which has a single-lobed far field pattern, is generally preferred to the (n-1).sup.th order supermode, which has a twin-lobed far field pattern. The design of such arrays can be modified to provide the zero order supermode with the lowest threshold, but it is generally found that operation of the zero order supermode tends to be unstable due primarily to self-focusing effects. The (n-1).sup.th order supermode is by contrast much more stable because it does not end to self-focus. It is therefore possible in principle to design laser arrays that should radiate stably in the (n-1).sup.th order supermode to high output powers. More detailed investigation of examples of such designs has, however, suggested that supermode stability is still sensitive to perturbations of the transverse built-in waveguide. In particular the small gain differences between the various supermodes with the highest gains can lead to solutions which oscillate between two or more supermodes.
An alternative design of injection laser array, hereinafter referred to as a Y-coupled array, is disclosed in United Kingdom patent specification GB No. 2163001A (inventors M. Taneya et al) in which a first set of n elements are coupled to a set of (n+1) elements by means of a set of (2n-1) Y couplers. An article entitled `0.degree. phase mode operation in phase-array laser diode with symmetrically branching waveguide`, by M. Taneya et al, appearing in Applied Physics Letters Vol. 47 No. 4 pp341-3 (15th Aug. 1975) describes the most basic form of Y-coupled array in which the first set of elements comprises only a single element (n=1), whereas a Y-coupled array with a larger number of elements is described by D. F. Welsh et al in an article entitled `In-phase emission from index guided laser array up to 400 mm` appearing in Electronics Letters Vol. 22 No. 6 pp293-4 (13th Mar. 1986). In such arrays the Y couplers serve to discriminate in favour of the zero order supermode because light propagating in phase into the two upper limits of an upright Y is guided out of the coupler via the stem of the Y whereas any out-of-phase component is beyond cut-off, and hence radiates out from the stem.
For any Y-coupled array for which n.gtoreq.2 it is to be noted that each of the intermediate elements of the set of (n+1) elements is coupled by Y couplers to two neighbouring elements in the set of n elements, whereas each of the outermost two elements of the array of (n+1) elements is coupled to only one element of the set of n elements, and therefore has only half the photon density of the intermediate elements. This will lead to a progressive difference in optical path length between that of the outermost elements and that of the intermediate elements as the drive current is increased. Eventually the phase difference that this engenders can be expected to feed back across the array and lead to a distorted radiated phase front, and even perhaps the generation of a higher order supermode. In principle it could be possible to modify the width and guiding strength of the two outermost elements so as to provide them with inreased photon density to match that of the intermediate elements, and so achieve carrier density clamping across the full width of the array above threshold. In practice, however, this would be difficult to achieve because it would involve knowledge of the precise guide strength remaining after taking into account the effects of carriers.